Heterogeneous network (HetNet) deployment is one important part of future radio networks in order to serve the increasing capacity and quality demands. A HetNet deployment consists of small cells overlaid in an existing macro network.
The small cells typically have lower output power compared to the macro cells and due to this the coverage of these cells are also smaller compared to the macro cells. A typical deployment for the small cells is therefore localized hotspots, such as a train station, where the traffic load is high in the small cells but the area is often covered by the surrounding Macro cell(s) also.
Overlapping coverage also means that the Macro cell will cause interference to the small cells if operating on the same carrier frequency. FIG. 1 shows an example of a HetNet scenario in a wireless communication network 1, where a wireless communication device 20 or user equipment (UE) near the cell border of its serving base station 10 experiences strong inter-cell interference from a Macro base station in a neighbour cell. Typically it is not optimal to switch off these interfering Macro cells since they are needed for maintaining full coverage and/or for covering traffic in e.g. high rise buildings surrounding the hotspot. By applying offsets (in power) at cell (re-)selection and handover, the coverage of the small cell can be made larger but with the drawback of increased downlink interference from the Macro cell. This is known as Cell Range Expansion (CRE).
To counteract severe interference scenarios it is possible to utilize coordination features, one of which is often referred to as Downlink Coordinated Multi-Point (DL CoMP), which perform fast (per scheduling interval, typically 1 ms) coordination between interfering cells, i.e. the transmission from one or more antennas are coordinated knowing the interference situation in the neighbour cells and its impact on a specific UE. See FIG. 2 for examples of coordination schemes. These schemes all have in common that they try to avoid (or at least predict) collisions when transmitting to several users in a cluster of several cells. Coordinated Link Adaptation tries to predict potential interfering sources and thereby improving the link adaptation accuracy, Dynamic Point Blanking prevents transmission in neighbour time/frequency grid to avoid inter-cell interference, Coordinated Beamforming tries to point a null (a beam with very low power) to primary victim while maintaining high throughput to selected user(s), and Joint Transmission tries to maximize the throughput to one or several users (UEs) while transmitting from several cells/antenna beams in parallel.
However, all the CoMP schemes described above require that the UEs perform measurements in order to select best beam/cell/transport format (user bitrate), and this results in increased processing need in the UE and network, and high signalling load over the radio interface. To reduce the effect of these drawbacks, it would be desirable to know when it is actually beneficial to use CoMP.